The MBTA has contracted with Springfield-based CRRC to design and manufacture a fleet replacement encompassing 152 new Orange Line vehicles and 252 new Red Line vehicles. MBTA photo

Six of the T’s new Orange Line cars, representing enough for one full train set, will remain out of service after the MBTA found they each had a small number of bolts improperly installed, MBTA General Manager Steve Poftak said Thursday.

The bulk of the newest additions to the T’s subway fleet – six Red Line cars and 50 Orange Line cars – have passed inspection and returned to service in the week since the agency pulled them from the tracks after a single braking unit failure disabled a train.

But Poftak told the MBTA’s board of directors that in the process of examining each of the Springfield-manufactured vehicles, workers found a total of 17 bolts out of the 1,584 inspected that were “out of the specified torque range.”

“In many cases, it was not visually obvious that something had occurred, but they were out of the specified torque range, so we are holding those seven cars right now for further analysis,” Poftak said.

One of the cars with improperly installed bolts was a Red Line vehicle, which has since been fixed and put back into service, Poftak said. The remaining Orange Line cars will return to service following additional analysis and a replacement of the faulty bolts.

Moving forward, Poftak said Chinese firm CRRC would change its assembly procedures as well as quality assurance and quality control measures in its Springfield plant where the vehicles are put together. The new vehicles will also be inspected every two weeks “until the investigation is concluded,” he wrote in his presentation to the board.

“We have put more money into cars and I don’t know if we’ll ever get that right, purchasing cars and getting it to run as soon as they get delivered,” House Speaker Ronald Mariano said Thursday morning when asked about the state of the MBTA.

The first new Orange Line trains returned to service Monday, four days after the MBTA halted running them.

“It’s more important that we get this right than avoid a couple of days of critical coverage,” Poftak said. “It’s surely more important to get this right.”